The first-order analytical approximation \(P_1\)  to transport theory is the diffusion theory [1]}, which is widely used to extract transport parameters from opaque media with isotropic scatterers (see Fig. REF (b)) and with negligible absorption.
For a slab shown in Fig. REF (a), this opaque configuration amounts to the thickness \(L\)  being much larger than the transport mean free path \({\ell }_{\textrm {tr}}\)  and much smaller than the absorption mean free path \({\ell }_{\textrm {abs}}\) :
\({\ell }_{\textrm {tr}} \ll L \ll {\ell }_{\textrm {abs}}\text{.}\) 
